This invention relates to surface mountable electronic devices which support an electronic element elastically within a case by sandwiching it between spring terminals. More in particular, this invention relates to such electronic devices suitable for containing a heat-emitting electronic element such as a thermistor or a varistor.
Surface mountable electronic devices are coming to be used widely as high-density electronic apparatus are being developed. Surface mountable electronic devices are usually formed by placing an electronic element on a connector such as an electrode land on a printed circuit board and connecting the electrodes of the electronic element electrically, for example by soldering, on the connector land.
FIG. 9 shows an example of such a surface mountable electronic device 51 disclosed in Japanese Patent Publication Tokkai 8-17603, having electrode caps 53 and 54 formed on both ends of a cylindrically shaped electronic element 52. The electrode caps 53 and 54 are attached to terminals 55 and 56 having end parts 55a and 56b bent parallel to the longitudinal direction of the element 52. When this is surface-mounted, these bent end parts 55a and 56b are placed on a printed circuit board 57 and a solder material 58 is applied. Although the electronic element 52 is cylindrically shaped and hence may tend to rotate easily, the terminals 55 and 56 serve to prevent such a rotary motion when the device 51 is surface-mounted by means of an ordinary mounting equipment.
FIG. 10 shows another example of a prior art surface mountable electronic device 61, having an electronic element 63 contained inside a case 62 of a synthetic resin material. The case 62 has a main body 62a with an opening at the top and a lid 62b serving to close up this opening. The element 63 has electrodes 63b and 63c formed on both surfaces of a disk-shaped main body 63b. A spring terminal 64 contacts the electrode 63a while the other electrode 63c is electrically connected to a terminal having a protrusion 65. The spring terminal 64 has an opposite end 64a pulled out of the case 62. The terminal having the protrusion 65 also has an opposite end part 65a pulled out of the case 62. These pulled-out end parts 64a and 65a are both soldered (indicated by numerals 66) to a printed circuit board 67. Such a device is considered useful for containing a heat-generating electronic element such as a thermistor or a varistor because the case 62 made of a resin material, the spring terminal 64 and the protrusion 65 serve to impede the propagation of generated heat to the solder material 66 at the junction with the circuit board 67.
As is well known, some electronic elements such as varistors and thermistors generate heat when in use and may reach a very high temperature while being mounted. In the case of the mounting shown in FIG. 9, it may be considered that the propagation of heat from the element 52 to the solder material 58 is impeded by the length of the terminals 55 and 56 but this is frequently not sufficient to prevent the deterioration of the circuit board 57 by heat if the element 52 becomes very hot. Even with a structure as shown in FIG. 10, the separation between the element 63 and the printed circuit board 67 is relatively small. If the temperature of element 63 exceeds 100.degree. C., for example, the junctures by the solder material 66 become fragile and/or the printed circuit board 67 starts to show burnt marks.